1. Field of the Invention
The present invention relates to electro-optical systems. More specifically, the present invention relates to biasing circuits for avalanche photodiodes.
2. Description of the Related Art
Photodiodes are p-n junction semiconductor diodes that are sensitive to electromagnetic radiation through a photovoltaic process. Incident photons with photon energies greater than the bandgap energy of the semiconductor can generate electron-hole pairs within the depletion layer of the diode, which drift under the influence of a reverse bias, generating a current that is added to the normal leakage current of the diode. Avalanche photodiodes (APDs) are operated at a sufficiently high reverse bias that the drift field within the semiconductor accelerates the photo-carriers, causing avalanche multiplication of the photocurrent through multiple impact ionizations.
The gain of an APD is the net increase in the number of signal carriers generated through this avalanche multiplication process. Avalanche multiplication is inherently noisy, as both minority and majority carriers contribute to the noise current, while only minority carriers contribute to the signal current. A problem associated with APDs is that as the background noise level varies, the optimum gain value varies. To maximize the sensitivity of an APD receiver, it is necessary to bias the APD devices at a reverse-bias voltage that optimizes the gain for lowest noise equivalent power (NEP) in the presence of solar and/or thermal background Shot noise, detector Shot noise due to detector leakage or dark current, and preamplifier Johnson or thermal noise.
Many approaches are known in the art for setting the bias of an APD detector. Some teach setting a fixed threshold, which provides no optimization for changes in temperature or noise level. Others teach methods for measuring the temperature and adjusting the bias according to a known gain vs. temperature characteristic, which are an improvement over the fixed bias approaches, but do not optimize the gain based on the ambient noise conditions. Others set the bias at a voltage that is a fixed fraction of the breakdown voltage or a fixed value below the breakdown voltage with similar results, but with the added risk of damage due to excessive detector current draw when the breakdown voltage is measured.
U.S. Pat. No. 4,236,069 entitled “AVALANCHE PHOTODIODE GAIN CONTROL SYSTEM” by Laughlin, appears to use an ambient noise measurement in the bias setting approach. Laughlin purports to teach an APD bias circuit that controls APD gain such that the output noise level of the APD device matches the preamplifier noise level with no APD device noise. This is a further improvement over the prior approaches inasmuch as it adjusts to varying noise levels. The Laughlin approach, however, is complex in its implementation, requiring a complicated preamplifier noise measurement circuit, and is not optimum across the range of detector types and anticipated noise conditions.
Hence, a need exists in the art for an improved system or method for setting the bias of an avalanche photodiode that optimizes signal-to-noise ratio over a broad range of detector types and noise conditions.